1. Field of the Invention
The present invention relates to high-field permanent magnets. More specifically, it relates to structures and techniques for augmenting a working magnetic field contained in a cavity of a permanent magnet.
2. Description of the Prior Art
High-remanence, high-coercivity, permanent-magnet materials, such as those of the rare-earth type, have improved so that it is now practical to make flux sources of extraordinary strength and compaction. Examples of high-intensity, compact permanent magnets may be found in the following references:
Leupold, U.S. Pat. No. 4,835,506, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source;"
Leupold, U.S. Pat. No. 4,837,542, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source for Producing a Uniform High Field;"
Leupold, U.S. Pat. No. 4,839,059, entitled "Clad Magic Ring Wigglers;"
Leupold, U.S. Pat. No. 5,103,200, entitled "High-Field Permanent Magnet Flux Source;"
Leupold, U.S. Statutory Invention Registration H591, entitled "Method of Manufacturing of a Magic Ring;"
Leupold et al., "Novel High-Field Permanent-Magnet Flux Sources," IEEE Transactions on Magnetics, vol. MAG-23, No. 5, pp. 3628-3629, Sept. 1987;
Leupold et al., "A Catalogue of Novel Permanent-Magnet Field Sources," Paper No. W3.2, 9th International Workshop on Rare-Earth Magnets and Their Applications, pp 109-123, Aug. 1987, Bad Soden, FRG;
Leupold et al., "Design applications of magnetic mirrors," Journal of Applied Physics, 63(8), Apr. 15, 1988, pp. 3987-3988;
Leupold et al., "Applications of yokeless flux confinement," Journal of Applied Physics, 64(10), Nov. 15, 1988, pp. 5994-5996; and
Abele et al., "A general method for flux confinement in permanent-magnet structures," Journal of Applied Physics, 64(10), Nov. 15, 1988, pp. 5988-5990.
Additionally, magnets of the type described herein may be found in the following co-pending U.S. patent applications that are incorporated herein by reference:
Ser. No. 654,476, filed Feb. 13, 1991, entitled "High-Power Electrical Machinery;"
Ser. No. 650,845, filed Feb. 5, 1991, entitled "High-Power Electrical Machinery with Toroidal Permanent Magnets;" and
Ser. No. 892,104, filed concurrently herewith, entitled "Magnetic Field Sources Having Non-Distorting Access Ports," Docket No.: CECOM 4666.
These references show a number of high-intensity permanent magnets having a variety of different compact shapes. In general, the magnets described in these references have a shell of magnetic material and a cavity in which a working field is located. Access ports of various sizes, shapes and locations pass through the shell and communicate with the cavity.
Salient among these magnets are cylindrical ("magic ring") and spherical ("magic sphere") magnetic shells in which the direction of magnetization changes as a function of a polar angle. These magnets produce in their cavities uniform, polar-axial transverse fields. Theoretically, there is no limit to the fields attainable in a cavity of this type if one is willing to employ enough magnetic material of sufficiently high coercivity to retain its magnetism in the face of the high distorting fields engendered by the structure.
In practice, it is difficult to produce a spherical or cylindrical shell having a remanence or remanent magnetization the direction of which continuously varies. Consequently, such shells are typically constructed from segments that are each uniformly magnetized. When nested, the segments form a magnetic shell. In the case of a segmented cylindrical shell, the angular direction of magnetization changes abruptly by 4 .pi./N between adjacent segments, where N is the number of nested segments.
A working field produced by a segmented shell suffers surprisingly little from the approximation by segmentation. For example, if a cylindrically shaped shell is divided into sixteen segments, it produces a magnetic field of over 97% of that produced by a continuous structure. Even with a coarse approximation of only eight segments, 90% of the ideal field is realized. Specifically, a segmented spherical shell having an outer radius of 3.3 centimeters that is made of a magnetic material having a remanence of ten kilogauss can produce a field of sixteen kilo-oerstead in a spherical cavity having a radius of only 1.0 centimeter. The shell would have a mass of only 1.1 kilograms. Similar performance is obtainable from cylindrical and hemispherical structures.
Although such high-intensity, compact magnets have served the purpose, those concerned with their development have recognized the need for structures and techniques that further increase magnetic intensity without appreciably increasing size and mass. The present invention fulfills this need.